The main protein component of the amyloid plaques found in the brain of Alzheimer's disease (AD) patients is Aβ, a 4 kDa peptide consisting of mainly forty and forty-two residues (Aβ1-40, Aβ1-42) being derived from the amyloid precursor protein (APP). APP can be cleaved at the N-terminus of Aβ by an enzyme called β-secretase generating a soluble APP and the C-terminal fragment A4CT (C99). This 99 residue long membrane protein A4CT (ref. 1) which is the direct precursor for Aβ contains the entire Aβ domain, the membrane domain and the cytoplasmic tail of APP. β-secretase is thought to be a protein called aspartyl protease 2 (Asp 2). Contradictory evidence exists as to whether a highly homologous protein, aspartyl protease 1 (Asp 1) also has β-secretase activity (Refs 2, 3). It is thought that Asp 1 may play a role in amyloidogenesis in peripheral rather than brain tissues.
Both C-terminal fragments of APP, A4CT and p3CT, are cleaved within the membrane domain by a γ-cleavage activity, thereby releasing Aβ and p3 into the medium (refs. 4, 5). In cells expressing wild type APP the site of γ-cleavage is mainly the peptide bond Val(40)-Ile(41) of A4CT and to a minor extent the bond Ala(42)-Thr(43). In cells expressing APP with the Familial AD linked mutations at Val 717 (based on APP770, Val 46 of A4CT) an increased γ-cleavage occurs behind Val(42), thus producing larger amounts of Aβ1-42 (ref. 6).
Several groups have attempted to develop transgenic mouse models that demonstrate convincing pathology indicative of alzheimers disease (AD) such as the PDAPP mice, Tg2576, TgAPP/Sw/1, etc (Reviewed in ref 7) However, no transgenic mouse models have yet been produced which overexpress human β-secretase (Asp 2). This enzyme is one of the two key enzymes that cleave Amyloid Precursor Protein to liberate the β-amyloid peptide, the major constituent of amyloid plaques in Alzhemier Disease brain. It is likely that such mice will develop convincing pathology as over-expression of this enzyme will increase the level of cleavage of Amyloid Precursor Protein at the β-site. Therefore this would represent a novel model of AD as all existing transgenic animal models rely upon the expression of mutant forms of human genes which are linked to early onset Alzheimer's Disease (Amyloid Precursor Protein and Presenilin-1). Such a model should represent a faithful model system for the production and deposition of amyloid deposits characterisitic of Alzheimer's disease.
Results obtained depend upon the source of the promoter and the protein coding sequence used. We have found that the use of a specific neuronal promoter will increase the possibility of the development of pathology that resembles the clinical situation.